Linux Partitions: A Primer

Hard Drive Basics

June 20, 2002

By
Dee-Ann LeBlanc

Most computer users are aware of the existence of their hard drive, even if they're not familiar with how it works. It's hard not to be, since the first things people tend to ask when you get a new computer are how much hard drive space you have, how fast the machine is, and how much memory is included. From there, if you're used to using Windows, then you've probably dealt with the installer letting you break your hard drive into "multiple drives," probably referred to as C:, E:, F:, and so on.

You may not realize it, but if you've done this, you've already created partitions. A partition is a virtual drive inside a drive, created through storing information about the drive's virtual layout in special locations on the drive itself. The system's BIOS and operating system(s) then utilize this information to determine where to look on the drive for boot instructions and data.

A hard drive may look like a short, rectangular box, but the data is stored on a round section that looks somewhat like a stack of records (see Figure 1). The way data gets written to and read from this drive involves drive heads, which are housed on the carriage assembly shown in the image. Each of these electromagnetic heads alters data on a hard drive by manipulating the magnetic media at such a miniscule level that the head turns on and off individual bits and bytes.

When a PC boots, once the BIOS finishes loading hardware information, it looks to the very first spot on the hard drive, which is referred to as the Master Boot Record (MBR). How do we find this spot? The data on a hard drive is stored in a series of rings called tracks, and the tracks are subsequently broken up into equally sized pieces called sectors (see Figure 2). Tracks are numbered from 0 on up, and the counting starts at the outermost ring. Sectors are numbered from 1 on up.

The MBR is on track 0, sector 1: the very first location on the drive. In this tiny spot on your hard drive, two key pieces of information live. The first is the data on how many partitions you've created on the drive, and their vital statistics. The second is vital to whether your machine will boot properly or not without a floppy disk: the MBR contains a pointer to the specific partition or hard drive that has the boot information.

If you're familiar with Linux installation, then you'll know that the boot loaders LILO and GRUB place information in the MBR.